Identification of cell proliferation zones, progenitor cells and a potential stem cell niche in the intervertebral disc region: a study in four species

Single-cell sequencing reveals cellular heterogeneity of nucleus pulposus in intervertebral disc degeneration

The nucleus pulposus (NP) plays a vital role in intervertebral disc degeneration (IVDD). Previous studies have revealed cellular heterogeneity in NP tissue during IVDD progression. However, the cellular and molecular alterations of diverse cell clusters during IVDD remain to be fully elucidated. NP tissues were isolated from patients with different grades of IVDD undergoing discectomy, and then subjected to single-cell RNA sequencing (scRNA-seq). Cell subsets were identified based on unbiased clustering of gene expression profiles. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were performed to determine the molecular features of diverse cell clusters. Monocle analysis was used to illustrate the differentiation trajectories of chondrocytes. Additionally, CellPhoneDB analysis revealed potential interactions between chondrocytes and other cells during IVDD. Based on the expression profiles of 47,610 individual cells, eight putative clusters including chondrocytes, endothelial cells, fibroblasts, macrophages, mural cells, osteoclasts, proliferating stromal cells and T cells were identified. The chondrocyte cluster was classified into three subsets, C1-C3, which were associated with stress-resistance, fibrosis and inflammatory responses, respectively. Pseudo-time trajectories suggested that chondrocytes gradually differentiated into fibroblasts during IVDD. Immune cells including cDC2s, macrophages and monocytes were identified. Further analysis showed that chondrocytes might communicate with immune cells via the MIF, TNFSF9, SPP1 and CCL4L2 signaling pathways. In addition, we found that invading endothelial cells might interact with chondrocytes through the COL4A1, CXCL12, VEGFA and SEMA3E signaling pathways. Our results reveal the cellular complexity and phenotypic characteristics of NP tissues at single-cell resolution, which will contribute to the in-depth investigation of preventative and regenerative strategies for IVDD.

Progenitor-like cells contributing to cellular heterogeneity in the nucleus pulposus are lost in intervertebral disc degeneration

The nucleus pulposus (NP) in the intervertebral disc (IVD) arises from embryonic notochord. Loss of notochordal-like cells in humans correlates with onset of IVD degeneration, suggesting that they are critical for healthy NP homeostasis and function. Comparative transcriptomic analyses identified expression of progenitor-associated genes (GREM1, KRT18, and TAGLN) in the young mouse and non-degenerated human NP, with TAGLN expression reducing with aging. Lineage tracing using Tagln-CreERt2 mice identified peripherally located proliferative NP (PeriNP) cells in developing and postnatal NP that provide a continuous supply of cells to the entire NP. PeriNP cells were diminished in aged mice and absent in puncture-induced degenerated discs. Single-cell transcriptomes of postnatal Tagln-CreERt2 IVD cells indicate enrichment for TGF-β signaling in Tagln descendant NP sub-populations. Notochord-specific removal of TGF-β/BMP mediator Smad4 results in loss of Tagln+ cells and abnormal NP morphologies. We propose Tagln+ PeriNP cells are potential progenitors crucial for NP homeostasis.

Spatially multicellular variability of intervertebral disc degeneration by comparative single-cell analysis

Previous studies have revealed cellular heterogeneity in intervertebral discs (IVDs). However, the cellular and molecular alteration patterns of cell populations during degenerative progression remain to be fully elucidated. To illustrate the cellular and molecular alteration of cell populations in intervertebral disc degeneration (IDD), we perform single cell RNA sequencing on cells from four anatomic sites of healthy and degenerative goat IVDs. EGLN3+ StressCs, TGFBR3+ HomCs and GPRC5A+ RegCs exhibit the characteristics associated with resistance to stress, maintaining homeostasis and repairing, respectively. The frequencies and signatures of these cell clusters fluctuate with IDD. Notably, the chondrogenic differentiation programme of PROCR+ progenitor cells is altered by IDD, while notochord cells turn to stemness exhaustion. In addition, we characterise CAV1+ endothelial cells that communicate with chondrocytes through multiple signalling pathways in degenerative IVDs. Our comprehensive analysis identifies the variability of key cell clusters and critical regulatory networks responding to IDD, which will facilitate in-depth investigation of therapeutic strategies for IDD.

The clock transcription factor BMAL1 is a key regulator of extracellular matrix homeostasis and cell fate in the intervertebral disc

The circadian clock in mammals temporally coordinates physiological and behavioural processes to anticipate daily rhythmic changes in their environment. Chronic disruption to circadian rhythms (e.g., through ageing or shift work) is thought to contribute to a multitude of diseases, including degeneration of the musculoskeletal system. The intervertebral disc (IVD) in the spine contains circadian clocks which control ∼6% of the transcriptome in a rhythmic manner, including key genes involved in extracellular matrix (ECM) homeostasis. However, it remains largely unknown to what extent the local IVD molecular clock is required to drive rhythmic gene transcription and IVD physiology. In this work, we identified profound age-related changes of ECM microarchitecture and an endochondral ossification-like phenotype in the annulus fibrosus (AF) region of the IVD in the Col2a1-Bmal1 knockout mice. Circadian time series RNA-Seq of the whole IVD in Bmal1 knockout revealed loss of circadian patterns in gene expression, with an unexpected emergence of 12 h ultradian rhythms, including FOXO transcription factors. Further RNA sequencing of the AF tissue identified region-specific changes in gene expression, evidencing a loss of AF phenotype markers and a dysregulation of ECM and FOXO pathways in Bmal1 knockout mice. Consistent with an up-regulation of FOXO1 mRNA and protein levels in Bmal1 knockout IVDs, inhibition of FOXO1 in AF cells suppressed their osteogenic differentiation. Collectively, these data highlight the importance of the local molecular clock mechanism in the maintenance of the cell fate and ECM homeostasis of the IVD. Further studies may identify potential new molecular targets for alleviating IVD degeneration.

Intervertebral disc degeneration-Current therapeutic options and challenges

Degeneration of the intervertebral disc (IVD) is a normal part of aging. Due to the spine's declining function and the development of pain, it may affect one's physical health, mental health, and socioeconomic status. Most of the intervertebral disc degeneration (IVDD) therapies today focus on the symptoms of low back pain rather than the underlying etiology or mechanical function of the disc. The deteriorated disc is typically not restored by conservative or surgical therapies that largely focus on correcting symptoms and structural abnormalities. To enhance the clinical outcome and the quality of life of a patient, several therapeutic modalities have been created. In this review, we discuss genetic and environmental causes of IVDD and describe promising modern endogenous and exogenous therapeutic approaches including their applicability and relevance to the degeneration process.

Stem cells and discogenic back pain

BACKGROUND

Chronic low back pain, common from the sixth decade, negatively impacts the quality of life of patients and health care systems. Recently, mesenchymal stem cells (MSCs) have been introduced in the management of degenerative discogenic pain. The present study summarizes the current knowledge on the effectiveness of MSCs in patients with discogenic back pain.

SOURCES OF DATA

We performed a systematic review of the literature following the PRISMA guidelines. We searched PubMed and Google Scholar database, and identified 14 articles about management of chronic low back pain with MSCs injection therapy. We recorded information on type of stem cells employed, culture medium, clinical scores and MRI outcomes.

AREAS OF AGREEMENT

We identified a total of 303 patients. Ten studies used bone marrow stem cells. In the other four studies, different stem cells were used (of adipose, umbilical, or chondrocytic origin and a pre-packaged product). The most commonly used scores were Visual Analogue Scale and Oswestry Disability Index.

AREAS OF CONTROVERSY

There are few studies with many missing data.

GROWING POINTS

The studies analysed demonstrate that intradiscal injections of MSCs are effective on discogenic low-back pain. This effect may result from inhibition of nociceptors, reduction of catabolism and repair of injured or degenerated tissues.

AREAS TIMELY FOR DEVELOPING RESEARCH

Further research should define the most effective procedure, trying to standardize a single method.

Potential Role for Stem Cell Regenerative Therapy as a Treatment for Degenerative Disc Disease and Low Back Pain: A Systematic Review

Back pain is the single leading cause of disability worldwide. Despite the prevalence and morbidity of lower back pain, we still lack a gold-standard treatment that restores the physiological function of degenerated intervertebral discs. Recently, stem cells have emerged as a promising strategy for regenerative therapy for degenerative disc disease. In this study, we review the etiology, pathogenesis, and developing treatment strategies for disc degeneration in low back pain with a focus on regenerative stem cell therapies. A systematic search of PubMed/MEDLINE/Embase/Clinical Trials.gov databases was conducted for all human subject abstracts or studies. There was a total of 10 abstracts and 11 clinical studies (1 RCT) that met the inclusion criteria. The molecular mechanism, approach, and progress of the different stem cell strategies in all studies are discussed, including allogenic bone marrow, allogenic discogenic cells, autologous bone marrow, adipose mesenchymal stem cells (MSCs), human umbilical cord MSC, adult juvenile chondrocytes, autologous disc derived chondrocytes, and withdrawn studies. Clinical success with animal model studies is promising; however, the clinical outcomes of stem cell regenerative therapy remain poorly understood. In this systematic review, we found no evidence to support its use in humans. Further studies on efficacy, safety, and optimal patient selection will establish whether this becomes a viable, non-invasive therapeutic option for back pain.

Spatiotemporal Characterization of Human Early Intervertebral Disc Formation at Single-Cell Resolution

The intervertebral disc (IVD) acts as a fibrocartilaginous joint to anchor adjacent vertebrae. Although several studies have demonstrated the cellular heterogeneity of adult mature IVDs, a single-cell transcriptomic atlas mapping early IVD formation is still lacking. Here, the authors generate a spatiotemporal and single cell-based transcriptomic atlas of human IVD formation at the embryonic stage and a comparative mouse transcript landscape. They identify two novel human notochord (NC)/nucleus pulposus (NP) clusters, SRY-box transcription factor 10 (SOX10)⁺ and cathepsin K (CTSK)⁺ , that are distributed in the early and late stages of IVD formation and they are validated by lineage tracing experiments in mice. Matrisome NC/NP clusters, T-box transcription factor T (TBXT)⁺ and CTSK⁺ , are responsible for the extracellular matrix homeostasis. The IVD atlas suggests that a subcluster of the vertebral chondrocyte subcluster might give rise to an inner annulus fibrosus of chondrogenic origin, while the fibroblastic outer annulus fibrosus preferentially expresseds transgelin and fibromodulin . Through analyzing intercellular crosstalk, the authors further find that notochordal secreted phosphoprotein 1 (SPP1) is a novel cue in the IVD microenvironment, and it is associated with IVD development and degeneration. In conclusion, the single-cell transcriptomic atlas will be leveraged to develop preventative and regenerative strategies for IVD degeneration.

Ccn2a-FGFR1-SHH signaling is necessary for intervertebral disc homeostasis and regeneration in adult zebrafish

Intervertebral disc (IVD) degeneration is the primary cause of back pain in humans. However, the cellular and molecular pathogenesis of IVD degeneration is poorly understood. This study shows that zebrafish IVDs possess distinct and non-overlapping zones of cell proliferation and cell death. We find that, in zebrafish, cellular communication network factor 2a (ccn2a) is expressed in notochord and IVDs. Although IVD development appears normal in ccn2a mutants, the adult mutant IVDs exhibit decreased cell proliferation and increased cell death leading to IVD degeneration. Moreover, Ccn2a overexpression promotes regeneration through accelerating cell proliferation and suppressing cell death in wild-type aged IVDs. Mechanistically, Ccn2a maintains IVD homeostasis and promotes IVD regeneration by enhancing outer annulus fibrosus cell proliferation and suppressing nucleus pulposus cell death through augmenting FGFR1-SHH signaling. These findings reveal that Ccn2a plays a central role in IVD homeostasis and regeneration, which could be exploited for therapeutic intervention in degenerated human discs.

The Influence of Intervertebral Disc Microenvironment on the Biological Behavior of Engrafted Mesenchymal Stem Cells

Intervertebral disc degeneration is the main cause of low back pain. Traditional treatment methods cannot repair degenerated intervertebral disc tissue. The emergence of stem cell therapy makes it possible to regenerate and repair degenerated intervertebral disc tissue. At present, mesenchymal stem cells are the most studied, and different types of mesenchymal stem cells have their own characteristics. However, due to the harsh and complex internal microenvironment of the intervertebral disc, it will affect the biological behaviors of the implanted mesenchymal stem cells, such as viability, proliferation, migration, and chondrogenic differentiation, thereby affecting the therapeutic effect. This review is aimed at summarizing the influence of each intervertebral disc microenvironmental factor on the biological behavior of mesenchymal stem cells, so as to provide new ideas for using tissue engineering technology to assist stem cells to overcome the influence of the microenvironment in the future.

Glis1 and oxaloacetate in nucleus pulposus stromal cell somatic reprogramming and survival

Regenerative medicine aims to repair degenerate tissue through cell refurbishment with minimally invasive procedures. Adipose tissue (FAT)-derived stem or stromal cells are a convenient autologous choice for many regenerative cell therapy approaches. The intervertebral disc (IVD) is a suitable target. Comprised of an inner nucleus pulposus (NP) and an outer annulus fibrosus (AF), the degeneration of the IVD through trauma or aging presents a substantial socio-economic burden worldwide. The avascular nature of the mature NP forces cells to reside in a unique environment with increased lactate levels, conditions that pose a challenge to cell-based therapies. We assessed adipose and IVD tissue-derived stromal cells through in vitro transcriptome analysis in 2D and 3D culture and suggested that the transcription factor Glis1 and metabolite oxaloacetic acid (OAA) could provide NP cells with survival tools for the harsh niche conditions in the IVD.

Researches on Stem and Progenitor Cells in Intervertebral Discs: An Analysis of the Scientific Landscape

Low back pain (LBP) is a common clinical symptom, and the prevalence is ranged from 60% to 70%. With the deepening of basic research, the development of intervertebral disc regeneration-oriented cell therapy, especially stem and progenitor cells therapy, showed good research prospects and was expected to become new methods of treatment for LBP. Our study is aimed at analyzing the scientific output of stem and progenitor cells in intervertebral discs and at driving future research into new publications. Researches focused on this file were searched from the Science Citation Index Expanded (SCI-E) of the Web of Science (WOS) core collection database and were screened according to inclusion criteria. We evaluated and visualized the results, including annual publications, citations, authors, organizations, countries, research directions, funds, and journals by bibliometric website, VOSviewer, and Citespace softwares on May 27, 2022. A total of 450 original articles and reviews were included, and the overall trend of the number of publications rapidly increased. In worldwide, China and the USA were the leading countries for research production. The retrieved 450 publications received 14322 citations, with an average of 31.83 citations and an H-index of 62. The most high-yield author, organization, country, research directions, funds, and journals were Chen QX from Zhejiang University, Zhejiang University, China, Cell Biology, National Natural Science Foundation of China, and Spine, respectively. Keywords cluster analysis showed the research hotspots in the future, including “human intervertebral disc”, “adipose-derived mesenchymal stem cell”, “intervertebral disc degeneration”, “degenerative disc model”, “nucleus pulposus regeneration”, “human cartilage”, “3d culture”, “shrinkage-free preparation”, and “polylactide disc”. Furthermore, with accumulating evidence demonstrating the role of stem and progenitor cells in intervertebral discs, “microenvironment”, “activation”, “intervertebral disc degeneration”, and “oxidative stress” are becoming the research frontiers and trends.

Expansion of FGFR3-positive nucleus pulposus cells plays important roles in postnatal nucleus pulposus growth and regeneration

BACKGROUND

Intervertebral disc degeneration (IVDD) can cause low back pain, a major public health concern. IVDD is characterized with loss of cells especially those in nucleus pulposus (NP), due to the limited proliferative potential and regenerative ability. Few studies, however, have been carried out to investigate the in vivo proliferation events of NP cells and the cellular contribution of a specific subpopulation of NP during postnatal growth or regeneration.

METHODS

We generated FGFR3-3*Flag-IRES-GFP mice and crossed FGFR3-CreERT2 mice with Rosa26-mTmG, Rosa26-DTA and Rosa26-Confetti mice, respectively, to perform inducible genetic tracing studies.

RESULTS

Expression of FGFR3 was found in the outer region of NP with co-localized expressions of proliferating markers. By fate mapping studies, FGFR3-positive (FGFR3⁺) NP cells were found proliferate from outer region to inner region of NP during postnatal growth. Clonal lineage tracing by Confetti mice and ablation of FGFR3^·+ NP cells by DTA mice further revealed that the expansion of the FGFR3⁺ cells was required for the morphogenesis and homeostasis of postnatal NP. Moreover, in degeneration and regeneration model of mouse intervertebral disc, FGFR3⁺ NP cells underwent extensive expansion during the recovery stage.

CONCLUSION

Our present work demonstrates that FGFR3⁺ NP cells are novel subpopulation of postnatal NP with long-existing proliferative capacity shaping the adult NP structure and participating in the homeostasis maintenance and intrinsic repair of NP. These findings may facilitate the development of new therapeutic approaches for IVD regeneration.

Extracellular Vesicles as an Emerging Treatment Option for Intervertebral Disc Degeneration: Therapeutic Potential, Translational Pathways, and Regulatory Considerations

Emergent approaches in regenerative medicine look toward the use of extracellular vesicles (EVs) as a next-generation treatment strategy for intervertebral disc (IVD) degeneration (IVDD) because of their ability to attenuate chronic inflammation, reduce apoptosis, and stimulate proliferation in a number of tissue systems. Yet, there are no Food and Drug Administration (FDA)-approved EV therapeutics in the market with an indication for IVDD, which motivates this article to review the current state of the field and provide an IVD-specific framework to assess its efficacy. In this systematic review, 29 preclinical studies that investigate EVs in relation to the IVD are identified, and additionally, the regulatory approval process is reviewed in an effort to accelerate emerging EV-based therapeutics toward FDA submission and timeline-to-market. The majority of studies focus on nucleus pulposus responses to EV treatment, where the main findings show that stem cell-derived EVs can decelerate the progression of IVDD on the molecular, cellular, and organ level. The findings also highlight the importance of the EV parent cell's pathophysiological and differentiation state, which affects downstream treatment responses and therapeutic outcomes. This systematic review substantiates the use of EVs as a promising cell-free strategy to treat IVDD and enhance endogenous repair.

Decoding the annulus fibrosus cell atlas by scRNA-seq to develop an inducible composite hydrogel: A novel strategy for disc reconstruction

Low back pain is one of the most serious public health problems worldwide and the major clinical manifestation of intervertebral disc degeneration (IVDD). The key pathological change during IVDD is dysfunction of the annulus fibrosus (AF). However, due to the lack of an in-depth understanding of AF biology, the methods to reconstruct the AF are very limited. In this study, the mice AF cell atlas were decoded by single-cell RNA sequencing to provide a guide for AF reconstruction. The results first identify a new population of AF cells, fibrochondrocyte-like AF cells, which synthesize both collagen I and collagen II and are potential functional cells for AF reconstruction. According to the dual features of the AF extracellular matrix, a composite hydrogel based on the acylation of methacrylated silk fibroin with methacrylated hyaluronic acid was produced. To obtain the ability to stimulate differentiation, the composite hydrogels were combined with a fibrochondrocyte-inducing supplement. Finally, reconstruction of the AF defects, by the novel AF stem cell-loaded composite hydrogel, could be observed, its amount of chondroid matrices recovered to 31.7% of AF aera which is significantly higher than that in other control groups. In summary, this study decodes the AF cell atlas, based on which a novel strategy for AF reconstruction is proposed.

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There are 10 populations of cells in the annulus fibrosus (AF), as decoded by single cell RNA sequencing.

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Lineage tracing shows the route of migration and differentiation of annulus fibrosus-derived stem cells (AFSCs).

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A new population of AF cells, fibrochondrocyte-like AF cells, was identified.

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Both the fibrinoid and chondroid matrices of AF are reconstructed by the novel AFSCs-loaded composite hydrogel.

Single-Cell Transcriptome Profiling Reveals Multicellular Ecosystem of Nucleus Pulposus during Degeneration Progression

Although degeneration of the nucleus pulposus (NP) is a major contributor to intervertebral disc degeneration (IVDD) and low back pain, the underlying molecular complexity and cellular heterogeneity remain poorly understood. Here, a comprehensive single-cell resolution transcript landscape of human NP is reported. Six novel human NP cells (NPCs) populations are identified by their distinct molecular signatures. The potential functional differences among NPC subpopulations are analyzed. Predictive transcripts, transcriptional factors, and signal pathways with respect to degeneration grades are explored. It is reported that fibroNPCs is the subpopulation for end-stage degeneration. CD90+NPCs are observed to be progenitor cells in degenerative NP tissues. NP-infiltrating immune cells comprise a previously unrecognized diversity of cell types, including granulocytic myeloid-derived suppressor cells (G-MDSCs). Integrin αM (CD11b) and oxidized low density lipoprotein receptor 1 (OLR1) as surface markers of NP-derived G-MDSCs are uncovered. The G-MDSCs are found to be enriched in mildly degenerated (grade II and III) NP tissues compared to severely degenerated (grade IV and V) NP tissues. Their immunosuppressive function and alleviation effects on NPCs' matrix degradation are revealed in vitro. Collectively, this study reveals the NPC-type complexity and phenotypic characteristics in NP, thereby providing new insights and clues for IVDD treatment.

OUP accepted manuscript

The treatment of intervertebral disc degeneration (IVDD) is still a huge challenge for clinical updated surgical techniques and basic strategies of intervertebral disc regeneration. Few studies have ever tried to combine surgery and cell therapy to bridge the gap between clinical and basic research. A prospective clinical study with a 72-month follow-up was conducted to assess the safety and feasibility of autologous discogenic cells transplantation combined with discectomy in the treatment of lumbar disc herniation (LDH) and to evaluate the regenerative ability of discogenic cells in IVDD. Forty patients with LDH who were scheduled to have discectomy enrolled in our study and were divided into the observed group (transplantation of autologous discogenic cells after discectomy) and control group (only-discectomy). Serial MRI and X-ray were used to evaluate the degenerative extent of index discs, and clinical scores were used to determine the symptomatic improvement. No adverse events were observed in the observed group, and seven patients in the control group underwent revisions. Both groups had significant improvement of all functional scores post-operatively, with the observed group improving more considerably at 36-month and 72-month follow-up. The height and water content of discs in both groups decreased significantly since 36 months post-op with the control group decreased more obviously. Discectomy combined with autologous discogenic cells transplantation is safe and feasible in the treatment of LDH. Radiological analysis demonstrated that discogenic cells transplantation could slow down the further degeneration of index discs and decrease the complications of discectomy.

Endogenous repair theory enriches construction strategies for orthopaedic biomaterials: a narrative review

The development of tissue engineering has led to new strategies for mitigating clinical problems; however, the design of the tissue engineering materials remains a challenge. The limited sources and inadequate function, potential risk of microbial or pathogen contamination, and high cost of cell expansion impair the efficacy and limit the application of exogenous cells in tissue engineering. However, endogenous cells in native tissues have been reported to be capable of spontaneous repair of the damaged tissue. These cells exhibit remarkable plasticity, and thus can differentiate or be reprogrammed to alter their phenotype and function after stimulation. After a comprehensive review, we found that the plasticity of these cells plays a major role in establishing the cell source in the mechanism involved in tissue regeneration. Tissue engineering materials that focus on assisting and promoting the natural self-repair function of endogenous cells may break through the limitations of exogenous seed cells and further expand the applications of tissue engineering materials in tissue repair. This review discusses the effects of endogenous cells, especially stem cells, on injured tissue repairing, and highlights the potential utilisation of endogenous repair in orthopaedic biomaterial constructions for bone, cartilage, and intervertebral disc regeneration.

The experimental study of regeneration of annulus fibrosus using decellularized annulus fibrosus matrix/poly(ether carbonate urethane)urea-blended fibrous scaffolds with varying elastic moduli

Although tissue engineering has attracted increasing attention for the treatment of degenerative intervertebral disc disease, the biochemical properties, structural organization, and mechanical characteristics of annulus fibrosus tissue have restricted progress. Differentiation of annulus fibrosus-derived stem cells (AFSCs) can be regulated by the elasticity of substrates such as poly(ether carbonate urethane)urea (PECUU). Decellularized annulus fibrosus matrix (DAFM) has good biocompatibility and biodegradability, making it suitable for cell adhesion, proliferation, and differentiation. In this study, we used a coaxial electrospinning method to synthesize DAFM/PECUU-blended fibrous scaffolds with elasticities approximating that of native inner and outer annulus fibrosus tissue. AFSCs cultured on DAFM/PECUU-blended fibrous scaffolds exhibited increased collagen type I gene expression with increasing elasticity of the scaffold material; notably, collagen type II and aggrecan gene expression exhibited the opposite trend. Regarding extracellular matrix secretion, collagen type I content gradually increased with substrate elasticity, while collagen type II and aggrecan contents decreased. In vivo evaluations employing magnetic resonance imaging, hematoxylin and eosin staining, and immunohistochemistry indicated that DAFM/PECUU-blended fibrous scaffolds could effectively repair defects of annulus fibrosus tissue. Our findings provide a theoretical and practical basis for the development of bionic annulus fibrosus tissue that closely mimics the biological properties, mechanical function, and matrix composition of native tissue.

Intervertebral Disc Stem/Progenitor Cells: A Promising "Seed" for Intervertebral Disc Regeneration

Intervertebral disc (IVD) degeneration is considered to be the primary reason for low back pain (LBP), which has become more prevalent from 21 century, causing an enormous economic burden for society. However, in spite of remarkable improvements in the basic research of IVD degeneration (IVDD), the effects of clinical treatments of IVDD are still leaving much to be desired. Accumulating evidence has proposed the existence of endogenous stem/progenitor cells in the IVD that possess the ability of proliferation and differentiation. However, few studies have reported the biological properties and potential application of IVD progenitor cells in detail. Even so, these stem/progenitor cells have been consumed as a promising cell source for the regeneration of damaged IVD. In this review, we will first introduce IVD, describe its physiology and stem/progenitor cell niche, and characterize IVDSPCs between homeostasis and IVD degeneration. We will then summarize recent studies on endogenous IVDSPC-based IVD regeneration and exogenous cell-based therapy for IVDD. Finally, we will discuss the potential applications and future developments of IVDSPC-based repair of IVD degeneration.

Regenerative Potential of Perichondrium: A Tissue Engineering Perspective

The clinical relevance of perichondrium was recognized more than a century ago. In children and adolescents, perichondrium is essential for the formation and growth of the cartilaginous part of craniofacial features and must be considered during reconstructive surgery in the head and neck area. Also in adults, perichondrium must be preserved during surgical intervention for adequate postoperative healing and cartilage maintenance. Furthermore, the regenerative function of perichondrium in the ribs enables the harvesting of the rib cartilage tissue for reconstruction of craniofacial features. With the advancement of tissue engineering, renewed attention has been focused on the perichondrium, because without this crucial tissue, the function of cartilage engineered for craniofacial reconstruction is incomplete and may not be suitable for long-term reconstructive goals. Furthermore, interest in the perichondrium was revived owing to its possible role as a microenvironment containing stem and progenitor cells. Here we will revisit seminal studies on the perichondrium and review the current literature to provide a holistic perspective on the importance of this tissue in the context of regenerative medicine. We will also highlight the functional significance of perichondrium for cartilage tissue engineering.

Intervertebral disc degeneration and regeneration: a motion segment perspective

Back and neck pain have become primary reasons for disability and healthcare spending globally. While the causes of back pain are multifactorial, intervertebral disc degeneration is frequently cited as a primary source of pain. The annulus fibrosus (AF) and nucleus pulposus (NP) subcomponents of the disc are common targets for regenerative therapeutics. However, disc degeneration is also associated with degenerative changes to adjacent spinal tissues, and successful regenerative therapies will likely need to consider and address the pathology of adjacent spinal structures beyond solely the disc subcomponents. This review summarises the current state of knowledge in the field regarding associations between back pain, disc degeneration, and degeneration of the cartilaginous and bony endplates, the AF-vertebral body interface, the facet joints and spinal muscles, in addition to a discussion of regenerative strategies for treating pain and degeneration from a whole motion segment perspective.

The effects of 3D culture on the expansion and maintenance of nucleus pulposus progenitor cell multipotency

INTRODUCTION

Low back pain (LBP) is a global health concern. Increasing evidence implicates intervertebral disk (IVD) degeneration as a major contributor. In this respect, tissue-specific progenitors may play a crucial role in tissue regeneration, as these cells are perfectly adapted to their niche. Recently, a novel progenitor cell population was described in the nucleus pulposus (NP) that is positive for Tie2 marker. These cells have self-renewal capacity and in vitro multipotency potential. However, extremely low numbers of the NP progenitors limit the feasibility of cell therapy strategies.

OBJECTIVE

Here, we studied the influence of the culture method and of the microenvironment on the proliferation rate and the differentiation potential of human NP progenitors in vitro.

METHOD

Cells were obtained from human NP tissue from trauma patients. Briefly, the NP tissue cells were cultured in two-dimensional (2D) (monolayer) or three-dimensional (3D) (alginate beads) conditions. After 1 week, cells from 2D or 3D culture were expanded on fibronectin-coated flasks. Subsequently, expanded NP cells were then characterized by cytometry and tri-lineage differentiation, which was analyzed by qPCR and histology. Moreover, experiments using Tie2+ and Tie2- NP cells were also performed.

RESULTS

The present study aims to demonstrate that 3D expansion of NP cells better preserves the Tie2+ cell populations and increases the chondrogenic and osteogenic differentiation potential compared to 2D expansion. Moreover, the cell sorting experiments reveal that only Tie2+ cells were able to maintain the pluripotent gene expression if cultured in 3D within alginate beads. Therefore, our results highly suggest that the maintenance of the cell's multipotency is mainly, but not exclusively, due to the higher presence of Tie2+ cells due to 3D culture.

CONCLUSION

This project not only might have a scientific impact by evaluating the influence of a two-step expansion protocol on the functionality of NP progenitors, but it could also lead to an innovative clinical approach.

A challenging playing field: Identifying the endogenous impediments to annulus fibrosus repair

Intervertebral disc (IVD) herniations, caused by annulus fibrosus (AF) tears that enable disc tissue extrusion beyond the disc space, are very prevalent, especially among adults in the third to fifth decade of life. Symptomatic herniations, in which the extruded tissue compresses surrounding nerves, are characterized by back pain, numbness, and tingling and can cause extreme physical disability. Patients whose symptoms persist after nonoperative intervention may undergo surgical removal of the herniated tissue via microdiscectomy surgery. The AF, however, which has a poor endogenous healing ability, is left unrepaired increasing the risk for re-herniation and pre-disposing the IVD to degenerative disc disease. The lack of understanding of the mechanisms involved in native AF repair limits the design of repair systems that overcome the impediments to successful AF restoration. Moreover, the complexity of the AF structure and the challenging anatomy of the repair environment represents a significant challenge for the design of new repair devices. While progress has been made towards the development of an effective AF repair technique, these methods have yet to demonstrate long-term repair and recovery of IVD biomechanics. In this review, the limitations of endogenous AF healing are discussed and key cellular events and factors involved are highlighted to identify potential therapeutic targets that can be integrated into AF repair methods. Clinical repair strategies and their limitations are described to further guide the design of repair approaches that effectively restore native tissue structure and function.

Current Progress in the Endogenous Repair of Intervertebral Disk Degeneration Based on Progenitor Cells

Intervertebral disk (IVD) degeneration is one of the most common musculoskeletal disease. Current clinical treatment paradigms for IVD degeneration cannot completely restore the structural and biomechanical functions of the IVD. Bio-therapeutic techniques focused on progenitor/stem cells, especially IVD progenitor cells, provide promising options for the treatment of IVD degeneration. Endogenous repair is an important self-repair mechanism in IVD that can allow the IVD to maintain a long-term homeostasis. The progenitor cells within IVD play a significant role in IVD endogenous repair. Improving the adverse microenvironment in degenerative IVD and promoting progenitor cell migration might be important strategies for implementation of the modulation of endogenous repair of IVD. Here, we not only reviewed the research status of treatment of degenerative IVD based on IVD progenitor cells, but also emphasized the concept of endogenous repair of IVD and discussed the potential new research direction of IVD endogenous repair.

Bleomycin induces fibrotic transformation of bone marrow stromal cells to treat height loss of intervertebral disc through the TGFβR1/Smad2/3 pathway

Background

Lower back pain is often accredited to loss of intervertebral disc (IVD) height and compromised spine stability as a result of intervertebral disc degeneration (IVDD). We aim to locally use bleomycin to induce the fibrotic transformation of bone marrow stromal cells (BMSCs) as a means to induce reparative fibrosis to slow down the height loss.

Methods

IVDs from patients were gathered for histological examination. The expression of the transforming growth factor beta 1 (TGF-β) signaling pathway was determined by qPCR and western blotting. Nucleus pulposus (NP) cells, annulus fibrosus (AF) cells, and the rats’ bone marrow stromal cells (BMSC) were cultured and their responsiveness to bleomycin was evaluated by Cell Counting Kit-8, comet assay, transwell migration, and wound healing assays. Rat IVDD models were created by puncture and rescued by bleomycin injection, and the effectiveness was evaluated by images (X-ray and MRI) and atomic force microscope.

Results

Histological examination showed increased levels of pro-fibrotic markers in IVDD tissues from patients. AF cells and BMSC cells were induced to adopt a pro-fibrotic phenotype with increased expression fibrotic markers Col1a1, Col3a1, and FSP1. The pro-fibrotic effect of bleomycin on AF cells and BMSCs was in part due to the activation of the TGFβ-TGFβR1-SMAD2/3 signaling pathway. Pharmacological inhibition or gene knock-down of TGFβR1 could mitigate the pro-fibrotic effects.

Conclusion

Locally, injection of bleomycin in rats’ IVD induced rapid fibrosis and maintained its height through the TGFβ-TGFβR1-SMAD2/3 signaling pathway.

MRI evaluation of the hydration status of non-pathological lumbar intervertebral discs in a pediatric population

INTRODUCTION

The intervertebral disc (IVD) is made up of the annulus fibrosus (AF) and the nucleus pulposus (NP) - an inert hydrated complex. The ability of the IVD to deform is correlated to that of the NP and depends on its hydration. As the IVD ages, its hydration decreases along with its ability to deform. In adolescent idiopathic scoliosis, one of the etiological hypotheses pertains to the IVD, thus making its condition relevant for the diagnosis and monitoring of this pathology.

HYPOTHESIS

IVD hydration depends on sex, age and spine level in an asymptomatic pediatric population. The corollary is data on a control group of healthy subjects.

MATERIAL AND METHODS

A cohort of 98 subjects with normal spine MRI was enrolled; their mean age was 13.3 years. The disc volume and hydration of each IVD was evaluated on T2-weighted MRI sequences, using previously validated image processing software. This evaluation focused on the lumbar spine, from the thoracolumbar junction to the lumbosacral junction. It was assumed that IVD hydration was related to the ratio of NP and AF volumes. A mixed multivariate linear analysis was used to explore the impact of age, sex and spinal level on disc hydration.

RESULTS

Disc hydration was higher overall in boys than in girls, but this difference was not significant. Hydration increased with age by +0.005 for each additional year (p=0.0213). Disc hydration appears to be higher at the thoracolumbar junction than the lumbar spine, although this difference was not significant.

CONCLUSION

Through this MRI study, we established a database of non-pathological lumbar disc hydration as a function of age, sex and spinal segment along with 95% confidence intervals.

LEVEL OF EVIDENCE

IV.

Serial blockface SEM suggests that stem cells may participate in adult notochord growth in an invertebrate chordate, the Bahamas lancelet

Background

The cellular basis of adult growth in cephalochordates (lancelets or amphioxus) has received little attention. Lancelets and their constituent organs grow slowly but continuously during adult life. Here, we consider whether this slow organ growth involves tissue-specific stem cells. Specifically, we focus on the cell populations in the notochord of an adult lancelet and use serial blockface scanning electron microscopy (SBSEM) to reconstruct the three-dimensional fine structure of all the cells in a tissue volume considerably larger than normally imaged with this technique.

Results

In the notochordal region studied, we identified 10 cells with stem cell-like morphology at the posterior tip of the organ, 160 progenitor (Müller) cells arranged along its surface, and 385 highly differentiated lamellar cells constituting its core. Each cell type could clearly be distinguished on the basis of cytoplasmic density and overall cell shape. Moreover, because of the large sample size, transitions between cell types were obvious.

Conclusions

For the notochord of adult lancelets, a reasonable interpretation of our data indicates growth of the organ is based on stem cells that self-renew and also give rise to progenitor cells that, in turn, differentiate into lamellar cells. Our discussion compares the cellular basis of adult notochord growth among chordates in general. In the vertebrates, several studies implied that proliferating cells (chordoblasts) in the cortex of the organ might be stem cells. However, we think it is more likely that such cells actually constitute a progenitor population downstream from and maintained by inconspicuous stem cells. We venture to suggest that careful searches should find stem cells in the adult notochords of many vertebrates, although possibly not in the notochordal vestiges (nucleus pulposus regions) of mammals, where the presence of endogenous proliferating cells remains controversial.

Sox9 deletion causes severe intervertebral disc degeneration characterized by apoptosis, matrix remodeling, and compartment-specific transcriptomic changes

SOX9 plays an important role in chondrocyte differentiation and, in the developing axial skeleton, maintains the notochord and the demarcation of intervertebral disc compartments. Diminished expression is linked to campomelic dysplasia, resulting in severe scoliosis and progressive disc degeneration. However, the specific functions of SOX9 in the adult spinal column and disc are largely unknown. Accordingly, employing a strategy to conditionally delete Sox9 in Acan-expressing cells (Acan^CreERT2Sox9^fl/fl), we delineated these functions in the adult intervertebral disc. Acan^CreERT2Sox9^fl/fl mice (Sox9^cKO) showed extensive and progressive remodeling of the extracellular matrix in nucleus pulposus (NP) and annulus fibrosus (AF), consistent with human disc degeneration. Progressive degeneration of the cartilaginous endplates (EP) was also evident in Sox9^cKO mice, and it preceded morphological changes seen in the NP and AF compartments. Fate mapping using tdTomato reporter, EdU chase, and quantitative immunohistological studies demonstrated that SOX9 is crucial for disc cell survival and phenotype maintenance. Microarray analysis showed that Sox9 regulated distinct compartment-specific transcriptomic landscapes, with prominent contributions to the ECM, cytoskeleton-related, and metabolic pathways in the NP and ion transport, the cell cycle, and signaling pathways in the AF. In summary, our work provides new insights into disc degeneration in Sox9^cKO mice at the cellular, molecular, and transcriptional levels, underscoring tissue-specific roles of this transcription factor. Our findings may direct future cell therapies targeting SOX9 to mitigate disc degeneration.

MSC-Derived Exosomes Protect Vertebral Endplate Chondrocytes against Apoptosis and Calcification via the miR-31-5p/ATF6 Axis

Apoptosis and calcification of endplate chondrocytes (EPCs) can exacerbate intervertebral disc degeneration (IVDD). Mesenchymal stem cell-derived exosomes (MSC-exosomes) are reported to have the therapeutic potential in IVDD. However, the effects and related mechanisms of MSC-exosomes on EPCs are still unclear. We aimed to investigate the role of MSC-exosomes on EPCs with a tert-butyl hydroperoxide (TBHP)-induced oxidative stress cell model and IVDD rat model. First, our study revealed that TBHP could result in apoptosis and calcification of EPCs, and MSC-exosomes could inhibit the detrimental effects. We also found that these protective effects were inhibited after miroRNA (miR)-31-5p levels were downregulated in MSC-exosomes. The target relationship between miR-31-5p and ATF6 was tested. miR-31-5p negatively regulated ATF6-related endoplasmic reticulum (ER) stress and inhibited apoptosis and calcification in EPCs. Our in vivo experiments indicated that sub-endplate injection of MSC-exosomes can ameliorate IVDD; however, after miR-31-5p levels were downregulated in MSC-exosomes, these protective effects were inhibited. In conclusion, MSC-exosomes reduced apoptosis and calcification in EPCs, and the underlying mechanism may be related to miR-31-5p/ATF6/ER stress pathway regulation.

Hypoxia Protects Rat Bone Marrow Mesenchymal Stem Cells Against Compression-Induced Apoptosis in the Degenerative Disc Microenvironment Through Activation of the HIF-1α/YAP Signaling Pathway

Stem cell therapy provides an attractive solution for intervertebral disc (IVD) degeneration. However, the degenerative microenvironment, characterized by excessive mechanical loading and hypoxia, remains an obstacle for the long-lasting survival of exogenous transplanted stem cells. Whether and how bone marrow mesenchymal stem cells (BMSCs) adapt to the hostile microenvironment remain unclear. In this study, CoCl₂ and mechanical compression were simultaneously used to simulate the hypoxic and overloaded microenvironment of IVDs in vitro. Compression had a proapoptotic effect through activation of the mitochondrial apoptotic pathway, while hypoxia exerted a prosurvival effect counteracting compression-induced apoptosis. Inhibiting the transcriptional activity of hypoxia inducible factor 1 subunit alpha (HIF-1α) by chetomin reversed the antiapoptotic effect of hypoxia. Furthermore, HIF-1α promoted dephosphorylation and activation of yes-associated protein (YAP) in hypoxic conditions. Conversely, both YAP inhibition and increased cell apoptosis were observed after inhibition through chetomin or YAP inhibitor verteporfin. Immunofluorescence staining and coimmunoprecipitation assays revealed that YAP could interact directly with HIF-1α and colocalize in the nucleus. Taken together, our results demonstrated that hypoxia protected BMSCs against compression-induced apoptosis in the degenerative disc microenvironment through activation of the HIF-1α/YAP signaling pathway. Thus, regulation of HIF-1α/YAP signaling might provide novel insights for promoting long-lasting BMSC survival and optimizing stem cell therapy for IVD degeneration.

Comparison of different methods for the isolation and purification of rat nucleus pulposus-derived mesenchymal stem cells

PURPOSE

Recently, nucleus pulposus-derived mesenchymal stem cells (NPMSCs) have been identified and have shown good prospects for the repair of degenerative intervertebral discs. However, there is no consensus about the methods for the isolation and purification of NPMSCs. Therefore, a reliable and efficient isolation and purification method is potentially needed. We aimed to compare different methods and to identify an optimal method for isolating and purifying NPMSCs.

METHODS

NPMSCs were isolated and purified using two common methods (a low-density culture (LD) method and a mesenchymal stem cell complete medium culture (MSC-CM) method) and two novel methods (a cloning cylinder (CC) method and a combination of the CC and MSC-CM methods (MSC-CM+CC)). The morphology, MSC-specific surface markers (CD44, CD73, CD90, CD105, CD34 and HLA-DR), multiple-lineage differentiation potential, colony formation ability, and stemness gene (Oct4, Nanog, and Sox2) expression were evaluated and compared.

RESULTS

NPMSCs isolated from nucleus pulposus (NP) tissues via the four methods met the criteria stated by the International Society of Cell Therapy (ISCT) for MSCs, including adherent growth ability, MSC-specific surface antigen expression, and multi-lineage differentiation potential. In particular, the MSC-CM+CC method yielded a relatively higher quality of NPMSCs in terms of cell surface markers, multiple-lineage differentiation potential, colony formation ability, and stemness gene expression.

CONCLUSIONS

Our results indicated that NPMSCs can be obtained via all four methods and that the MSC-CM+CC method is more reliable and efficient than the other three methods. The findings from this study provide an alternative option for isolating and purifying NPMSCs.

Controlled release of biological factors for endogenous progenitor cell migration and intervertebral disc extracellular matrix remodelling

The recent description of resident stem/progenitor cells in degenerated intervertebral discs (IVDs) supports the notion that their regenerative capacities could be harnessed to stimulate endogenous repair of the nucleus pulposus (NP). In this study, we developed a delivery system based on pullulan microbeads (PMBs) for sequential release of the chemokine CCL-5 to recruit these disc stem/progenitor cells to the NP tissue, followed by the release of the growth factors TGF-β1 and GDF-5 to induce the synthesis of a collagen type II- and aggrecan-rich extracellular matrix (ECM). Bioactivity of released CCL5 on human adipose-derived stem cells (hASCs), selected to mimic disc stem/progenitors, was demonstrated using a Transwell® chemotaxis assay. The regenerative effects of loaded PMBs were investigated in ex vivo spontaneously degenerated ovine IVDs. Fluorescent hASCs were seeded on the top cartilaginous endplates (CEPs); the degenerated NPs were injected with PMBs loaded with CCL5, TGF-β1, and GDF-5; and the IVDs were then cultured for 3, 7, and 28 days to allow for cell migration and disc regeneration. The PMBs exhibited sustained release of biological factors for 21 days. Ex vivo migration of seeded hASCs from the CEP toward the NP was demonstrated, with the cells migrating a significantly greater distance when loaded PMBs were injected (5.8 ± 1.3 mm vs. 3.5 ± 1.8 mm with no injection of PMBs). In ovine IVDs, the overall NP cellularity, the collagen type II and the aggrecan staining intensities, and the Tie2+ progenitor cell density in the NP were increased at day 28 compared to the control groups. Considered together, PMBs loaded with CCL5/TGF-β1/GDF-5 constitute an innovative and promising strategy for controlled release of growth factors to promote cell recruitment and extracellular matrix remodelling.

Stem Cell Senescence: the Obstacle of the Treatment of Degenerative Disk Disease

Intervertebral disc (IVD) has a pivotal role in the maintenance of flexible motion. IVD degeneration is one of the primary causes of low back pain and disability, which seriously influences patients' health, and increases the family and social economic burden. Recently, stem cell therapy has been proven to be more effective on IVD degeneration disease. However, stem cell senescence is the limiting factor in the IVD degeneration treatment. Senescent stem cells have a negative effect on the self-repair on IVD degeneration. In this review, we delineate that the factors such as telomerase shortening, DNA damage, oxidative stress, microenvironment and exosomes will induce stem cell aging. Recent studies tried to delay the aging of stem cells by regulating the expression of aging-related genes and proteins, changing the activity of telomerase, improving the survival microenvironment of stem cells and drug treatment. Understanding the mechanism of stem cell aging and exploring new approaches to delay or reverse stem cell aging asks for research on the repair of the degenerated disc.

Proliferation, Migration, and ECM Formation Potential of Human Annulus Fibrosus Cells Is Independent of Degeneration Status

OBJECTIVE

The objective was to evaluate the proliferating, migratory and extracellular matrix (ECM) forming potential of annulus fibrosus cells derived from early (edAFC) or advanced (adAFC) degenerative tissue and their usability as a possible cell source for regenerative approaches for AF closure.

DESIGN

EdAFC (n = 5 Pfirrman score of 2-3) and adAFC (n = 5 Pfirrman score of 4-5) were isolated from tissue of patients undergoing spine stabilizing surgery. Cell migration on stimulation with human serum (HS), platelet-rich plasma (PRP), and transforming growth factor β-3 (TGFB3) was assessed by migration assay and proliferation was assessed on stimulation with HS. Induction of ECM synthesis was evaluated by gene expression analysis of AF-related genes in three-dimensional scaffold cultures that have been stimulated with 5% PRP or 10 ng/mL TGFB3 and histologically by collagen type I, type II, alcian blue, and safranin-O staining.

RESULTS

EdAFC and adAFC were significantly attracted by 10% HS and 5% PRP. Additionally, both cell groups proliferated under stimulation with HS. Stimulation with 10 ng/mL TGFB3 showed significant induction of gene expression of collagen type II and aggrecan, while 5% PRP decreased the expression of collagen type I. Both cell groups showed formation of AF-like ECM after stimulation with TGFB3, whereas stimulation with PRP did not.

CONCLUSIONS

Our study demonstrated that AF cells retain their potential for proliferation, migration, and ECM formation independent of the degeneration status of the tissue. Proliferation, migration, and ECM synthesis of the endogenous AF cells can be supported by different supplements. Hence, endogenous AF cells might be a suitable cell source for a regenerative repair approaches.

Current Treatment Options for Intervertebral Disc Pathologies

The complex structure of the intervertebral disc within the spine is well suited to its mechanical function. However, it is also prone to degeneration, which is associated with various clinical symptoms and conditions, ranging from disc herniation to back pain to spinal stenosis. Most patients' conditions are managed conservatively but a small proportion progress to having surgery. This may be decompression (to remove tissue such as the disc, bone, or hypertrophic ligaments impinging on nerves) or fusion of the normally mobile intervertebral joint to immobilize it and so reduce pain. These used to involve fairly major surgical procedures, but in the past decade there has been much progress to make the surgery more refined and less invasive, for example using endoscopic approaches. Simultaneously, the research world has been studying and developing tissue engineering and cellular techniques for attempting to regenerate the intervertebral disc, whether simply the central nucleus pulposus or a complete intricate assembly to replicate the native structure of this and the surrounding annulus fibrosus, cartilage endplate, and bone. To date, none of the complex entities have been trialed, while cellular approaches are easier to utilize, have progressed to clinical trials, and may offer a better solution.

Is Osteogenic Differentiation of Human Nucleus Pulposus Cells a Possibility for Biological Spinal Fusion?

OBJECTIVE

The purpose of this study was to investigate whether a simple, biologically robust method for inducing calcification of degenerate intervertebral discs (IVD) could be developed to provide an alternative treatment for patients requiring spinal fusion.

DESIGN

Nucleus pulposus (NP) cells isolated from 14 human IVDs were cultured in monolayer and exposed to osteogenic medium, 1,25-dihydroxyvitamin D₃ (VitD₃), parathyroid hormone (PTH), and bone morphogenic proteins (BMPs) 2/7 to determine if they could become osteogenic. Similarly explant cultures of IVDs from 11 patients were cultured in osteogenic media with and without prior exposure to VitD₃ and BMP-2. Osteogenic differentiation was assessed by alkaline phosphatase activity and areas of calcification identified by alizarin red or von Kossa staining. Expression of osteogenic genes during monolayer culture was determined using polymerase chain reaction and explant tissues assessed for BMP inhibitors. Human bone marrow-derived mesenchymal stromal cells (MSCs) were used for comparison.

RESULTS

Standard osteogenic media was optimum for promoting mineralization by human NP cells in monolayer. Some osteogenic differentiation was observed with 10 nM VitD₃, but none following application of PTH or BMPs. Regions of calcification were detected in 2 of the eleven IVD tissue explants, one cultured in osteogenic media and one with the addition of VitD₃ and BMP-2.

CONCLUSIONS

Human NP cells can become osteogenic in monolayer and calcification of the extracellular matrix can also occur, although not consistently. Inhibitory factors within either the cells or the extracellular matrix may hinder osteogenesis, indicating that a robust biological fusion at this time requires further optimization.

A Membranome-Centered Approach Defines Novel Biomarkers for Cellular Subtypes in the Intervertebral Disc

OBJECTIVE

Lack of specific marker-sets prohibits definition and functional distinction of cellular subtypes in the intervertebral disc (IVD), such as those from the annulus fibrosus (AF) and the nucleus pulposus (NP).

DESIGN

We recently generated immortalized cell lines from human NP and AF tissues; these comprise a set of functionally distinct clonal subtypes. Whole transcriptome analyses were performed of 12 phenotypically distinct clonal cell lines (4× NP-Responder, 4× NP-nonResponder, 2× AF-Sheet forming, and 2× AF-nonSheet forming). Data sets were filtered for membrane-associated marker genes and compared to literature.

RESULTS

Comparison of our immortal cell lines to published primary NP, AF, and articular chondrocytes (AC) transcriptome datasets revealed preservation of AF and NP phenotypes. NP-specific membrane-associated genes were defined by comparison to AF cells in both the primary dataset (46 genes) and immortal cell-lines (161 genes). Definition of AF-specific membrane-associated genes yielded 125 primary AF cell and 92 immortal cell-line markers. Overlap between primary and immortal NP cells yielded high-confidence NP-specific marker genes for NP-R (CLDN11, TMEFF2, CA12, ANXA2, CD44) and NP-nR (EFNA1, NETO2, SLC2A1). Overlap between AF and immortal AF subtypes yielded specific markers for AF-S (COLEC12, LPAR1) and AF-nS (CHIC1).

CONCLUSIONS

The current study provides a reference platform for preclinical evaluation of novel membrane-associated cell type-specific markers in the IVD. Future research will focus on their biological relevance for IVD function in development, homeostasis, and degenerate conditions.

Pathomechanism of intervertebral disc degeneration

Intervertebral disc degeneration (IDD) is the main contributor to low back pain, which is a leading cause of disability worldwide. Although substantial progress has been made in elucidating the molecular mechanisms of IDD, fundamental and long-lasting treatments for IDD are still lacking. With increased understanding of the complex pathomechanism of IDD, alternative strategies for treating IDD can be discovered. A brief overview of the prevalence and epidemiologic risk factors of IDD is provided in this review, followed by the descriptions of anatomic, cellular, and molecular structure of the intervertebral disc as well as the molecular pathophysiology of IDD. Finally, the recent findings of intervertebral disc progenitors are reviewed and the future perspectives are discussed.

IVD progenitor cells: a new horizon for understanding disc homeostasis and repair

Intervertebral disc (IVD) degeneration is associated with low back pain. In IVDs, a high mechanical load, high osmotic pressure and hypoxic conditions create a hostile microenvironment for resident cells. How IVD homeostasis and function are maintained under stress remains to be understood; however, several research groups have reported isolating native endogenous progenitor-like or otherwise proliferative cells from the IVD. The isolation of such cells implies that the IVD might contain a quiescent progenitor-like population that could be activated for IVD repair and regeneration. Increased understanding of endogenous disc progenitor cells will improve our knowledge of IVD homeostasis and, when combined with tissue engineering techniques, might hold promise for future therapeutic applications. In this Review, the characteristics of progenitor cells in different IVD compartments are discussed, as well as the potency of different cell populations within the IVD. The stem cell characteristics of these cells are also compared with those of mesenchymal stromal cells. On the basis of existing evidence, whether and how IVD degeneration and the hostile microenvironment might affect endogenous progenitor cell function are considered, and ways to channel the potential of these cells for IVD repair are suggested.

Leptin receptor-expressing cells represent a distinct subpopulation of notochord-derived cells and are essential for disc homoeostasis

Background/objective

Intervertebral disc degeneration (IDD) remains to be an intractable clinical challenge. Although IDD is characterised by loss of notochordal cells (NCs) and dysfunction of nucleus pulposus (NP) cells, little is known about the origin, heterogeneity, fate and maintenance of NCs and NP cells, which further stunts the therapeutic development. Thus, effective tools to spatially and temporally trace specific cell lineage and clarify cell functions in intervertebral disc (IVD) development and homoeostasis are urgently required.

Methods

In this study, NP specimens were obtained from 20 patients with degenerative disc disease or scoliosis. LepR-Cre mice was crossed with R26R-Tdtomato mice to generate LepR-Cre; R26R-Tdtomato mice, which enabled fate-mapping of NPs from embryo stage to late adult. LMNA G609G/G609G mice was used to determine the effect of premature-aging induced IDD on LepR NPs. X-ray imaging was used to measure lumber disc height of mice.

Results

Here, we provide the first evidence that the leptin receptor (LepR) is preferentially expressed in NCs at embryonic stages and notochord-derived cells in the postnatal IVD. By using R26R-Tdtomato fluorescent reporter mice, we systematically analysed the specificity of activity and targeting efficiency of leptin receptor-Cre (LepR-Cre) in IVD tissues from the embryonic stage E15.5 to 6-month-old LepR-Cre; Rosa26-Tdtomato (R26R-Tdtomato) mice. Specifically, LepR-Cre targets a distinct subpopulation of notochord-derived cells closely associated with disc homoeostasis. The percentage of LepR-expressing NP cells markedly decreases in the postnatal mouse IVD and, more importantly, in the human IVD with the progression of IDD. Moreover, both spine instability-induced and premature ageing-induced IDD mouse models display the phenotype of IDD with decreased percentage of LepR-expressing NP cells. These findings uncover a potential role of LepR-expressing notochord-derived cells in disc homoeostasis and open the gate for therapeutically targeting the NP cell subpopulation.

Conclusion

In conclusion, our data prove LepR-Cre mice useful for mapping the fate of specific subpopulations of IVD cells and uncovering the underlying mechanisms of IDD.

The translational potential of this article

The translation potential of article is that we first identified LepR as a candidate marker of subpopulation of nucleus pulposus (NP) cells and provided LepR as a potential target for the treatment of intervertebral disc degeneration (IDD), which have certain profound significance.

Neonatal annulus fibrosus regeneration occurs via recruitment and proliferation of Scleraxis-lineage cells

Intervertebral disc (IVD) injuries are a cause of degenerative changes in adults which can lead to back pain, a leading cause of disability. We developed a model of neonatal IVD regeneration with full functional restoration and investigate the cellular dynamics underlying this unique healing response. We employed genetic lineage tracing in mice using Scleraxis (Scx) and Sonic hedgehog (Shh) to fate-map annulus fibrosus (AF) and nucleus pulposus (NP) cells, respectively. Results indicate functional AF regeneration after severe herniation injury occurs in neonates and not adults. AF regeneration is mediated by Scx-lineage cells that lose ScxGFP expression and adopt a stem/progenitor phenotype (Sca-1, days 3–14), proliferate, and then redifferentiate towards type I collagen producing, ScxGFP+ annulocytes at day 56. Non Scx-lineage cells were also transiently observed during neonatal repair, including Shh-lineage cells, macrophages, and myofibroblasts; however, these populations were no longer detected by day 56 when annulocytes redifferentiate. Overall, repair did not occur in adults. These results identify an exciting cellular mechanism of neonatal AF regeneration that is predominantly driven by Scx-lineage annulocytes.

Regulation of terminal hypertrophic chondrocyte differentiation in Prmt5 mutant mice modeling infantile idiopathic scoliosis

Idiopathic scoliosis (IS) is the most common type of musculoskeletal defect affecting children worldwide, and is classified by age of onset, location and degree of spine curvature. Although rare, IS with onset during infancy is the more severe and rapidly progressive form of the disease, associated with increased mortality due to significant respiratory compromise. The pathophysiology of IS, in particular for infantile IS, remains elusive. Here, we demonstrate the role of PRMT5 in the infantile IS phenotype in mouse. Conditional genetic ablation of PRMT5 in osteochondral progenitors results in impaired terminal hypertrophic chondrocyte differentiation and asymmetric defects of endochondral bone formation in the perinatal spine. Analysis of these several markers of endochondral ossification revealed increased type X collagen (COLX) and Ihh expression, coupled with a dramatic reduction in Mmp13 and RUNX2 expression, in the vertebral growth plate and in regions of the intervertebral disc in the Prmt5 conditional mutant mice. We also demonstrate that PRMT5 has a continuous role in the intervertebral disc and vertebral growth plate in adult mice. Altogether, our results establish PRMT5 as a critical promoter of terminal hypertrophic chondrocyte differentiation and endochondral bone formation during spine development and homeostasis.

This article has an associated First Person interview with the first author of the paper.

Summary: Loss of Prmt5 in osteochondral progenitors impairs terminal hypertrophic chondrocyte differentiation, leading to defects in endochondral bone formation and models infantile idiopathic scoliosis in mouse.

Stem cell therapy in discogenic back pain

Chronic low back pain has both substantial social and economic impacts on patients and healthcare budgets. Adding to the magnitude of the problem is the difficulty in identifying the exact causes of disc degeneration with modern day diagnostic and imaging techniques. With that said, current non-operative and surgical treatment modalities for discogenic low back pain fails to meet the expectations in many patients and hence the challenge. The objective for newly emerging stem cell regenerative therapy is to treat degenerative disc disease (DDD) by restoring the disc's cellularity and modulating the inflammatory response. Appropriate patient selection is crucial for the success of stem cell therapy. Regenerative modalities for discogenic pain currently focus on the use of either primary cells harvested from the intervertebral discs or stem cells from other sources whether autogenic or allogenic. The microenvironment in which stem cells are being cultured has been recognized to play a crucial role in directing or maintaining the production of the desired phenotypes and may enhance their regenerative potential. This has led to a more specific focus on innovating more effective culturing techniques, delivery vehicles and scaffolds for stem cell application. Although stem cell therapy might offer an attractive alternative treatment option, more clinical studies are still needed to establish on the safety and feasibility of such therapy. In this literature review, we aim to present the most recent in vivo and in vitro studies related to the use of stem cell therapy in the treatment of discogenic low back pain.

Effects of Stromal Cell-Derived Factor-1α Secreted in Degenerative Intervertebral Disc on Activation and Recruitment of Nucleus Pulposus-Derived Stem Cells

Stromal cell-derived factor-1α (SDF-1α) plays a significant role in mobilizing and recruiting mesenchymal stem cells (MSCs) to the sites of injury. This study investigated the potential of SDF-1α released in the degenerative intervertebral disc (IVD) to activate and recruit endogenous nucleus pulposus-derived stem cells (NPSCs) for regeneration in situ. We found SDF-1α was highly expressed and secreted by the native disc cells when cultured in the proinflammatory mediators in vitro mimicking the degenerative settings. Immunohistochemical staining also showed that the expression level of SDF-1α was significantly higher in the degenerative group compared to that in the normal group. In addition to enhancement of viability, SDF-1α significantly increased the number of NPSCs migrating into the center of the nucleotomized bovine IVD ex vivo. After the systemic delivery of exogenous PKH26-labelled NPSCs into the rats in vivo, there was a significant difference in the distribution of the migrated cells between the normal and the degenerative IVDs, which might be caused by the different expression levels of SDF-1α. However, blocking CXC chemokine receptor 4 (CXCR4) with AMD3100 effectively abrogated SDF-1α-stimulated proliferation and migration. Taken together, SDF-1α may be a key chemoattractant that is highly produced in response to the degenerative changes, which can be used to enhance the proliferation and recruitment of endogenous stem cells into the IVDs. These findings may be of importance for understanding IVD regenerative mechanisms and development of regenerative strategies in situ for IVD degeneration.

The cell-based approach in neurosurgery: ongoing trends and future perspectives

OBJECTIVE

Examination of the current trends and future perspectives of the cell-based therapies in neurosurgery.

METHODS

A PubMed/MEDLINE-based systematic review has been performed combining the main Medical Subject Headings (MeSH) regarding the cell- and tissue-based therapies with the "Brain", "Spinal Cord", "Spine" and "Skull" MeSH terms. Only articles in English published in the last 10 years and pertinent to neurosurgery have been selected.

RESULTS

A total of 1,173 relevant articles have been chosen. Somatic cells and gene-modification technologies have undergone the greatest development. Immunotherapies and gene therapies have been tested for the cure of glioblastoma, stem cells mainly for brain and spinal cord traumatic injuries. Stem cells have also found a rationale in the treatment of the cranial and spinal bony defects, and of the intervertebral disc degeneration, as well.Most of the completed or ongoing trials concerning the cell-based therapies in neurosurgery are on phase 2. Future perspectives involve the need to overcome issues related to immunogenicity, oncogenicity and routes for administration. Refinement and improvement of vector design and delivery are required within the gene therapies.

CONCLUSION

The last decade has been characterised by a progressive evolution of neurosurgery from a purely mechanical phase to a new biological one. This trend has followed the rapid and parallel development of translational medicine and nanotechnologies.The introduction of new technologies, the optimisation of the already existing ones, and the reduction of costs are among the main challenges of the foreseeable future.

Fluorescence-Activated Cell Sorting Is More Potent to Fish Intervertebral Disk Progenitor Cells Than Magnetic and Beads-Based Methods

Low back pain related to intervertebral disk (IVD) degeneration has a major socioeconomic impact on our aging society. Therefore, stem cell therapy to activate self-repair of the IVD remains an exciting treatment strategy. In this respect, tissue-specific progenitors may play a crucial role in IVD regeneration, as these cells are perfectly adapted to this niche. Such a rare progenitor cell population residing in the nucleus pulposus (NP) (NP progenitor cells [NPPCs]) was found positive for the angiopoietin-1 receptor (Tie2+), and was demonstrated to possess self-renewal capacity and in vitro multipotency. Here, we compared three sorting protocols; that is, fluorescence-activated cell sorting (FACS), magnetic-activated cell sorting (MACS), and a mesh-based label-free cell sorting system (pluriSelect), with respect to cell yield, potential to form colonies (colony-forming units), and in vitro functional differentiation assays for tripotency. The aim of this study was to demonstrate the efficiency of three widespread cell sorting methods for picking rare cells (<5%) and how these isolated cells then behave in downstream functional differentiation in adipogenesis, osteogenesis, and chondrogenesis. The cell yields among the isolation methods differed widely, with FACS presenting the highest yield (5.0% ± 4.0%), followed by MACS (1.6% ± 2.9%) and pluriSelect (1.1% ± 1.0%). The number of colonies formed was not significantly different between Tie2+ and Tie2- NPPCs. Only FACS was able to separate into two functionally different populations that showed trilineage multipotency, while MACS and pluriSelect failed to maintain a clear separation between Tie2+ and Tie2- populations in differentiation assays. To conclude, the isolation of NPPCs was possible with all three sorting methods, while FACS was the preferred technique for separation of functional Tie2+ cells. Impact Statement Tissue-specific progenitor cells such as nucleus pulposus progenitor cells of the IVD could become an ultimate cell source for tissue engineering strategies as these cells are presumably best adapted to the tissue's microenvironment. Fluorescence-activated cell sorting seemed to outcompete magnetic-activated cell sorting and pluriSelect concerning selecting a rare cell population from IVD tissue as could be demonstrated by improved cell yield and functional differentiation assays.

Lessons learned from intervertebral disc pathophysiology to guide rational design of sequential delivery systems for therapeutic biological factors

Intervertebral disc (IVD) degeneration has been associated with low back pain, which is a major musculoskeletal disorder and socio-economic problem that affects as many as 600 million patients worldwide. Here, we first review the current knowledge of IVD physiology and physiopathological processes in terms of homeostasis regulation and consecutive events that lead to tissue degeneration. Recent progress with IVD restoration by anti-catabolic or pro-anabolic approaches are then analyzed, as are the design of macro-, micro-, and nano-platforms to control the delivery of such therapeutic agents. Finally, we hypothesize that a sequential delivery strategy that i) firstly targets the inflammatory, pro-catabolic microenvironment with release of anti-inflammatory or anti-catabolic cytokines; ii) secondly increases cell density in the less hostile microenvironment by endogenous cell recruitment or exogenous cell injection, and finally iii) enhances cellular synthesis of extracellular matrix with release of pro-anabolic factors, would constitute an innovative yet challenging approach to IVD regeneration.